This application relates to wireless communication systems and techniques based on orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA).
Wireless communication systems use electromagnetic waves to communicate with wireless communication devices located within cells of coverage areas of the systems. A radio spectral range or band designated or allocated for a wireless communication service or a particular class of wireless services may be divided into different radio carrier frequencies for generating different communication frequency channels. This use of different frequencies for different communication channels may be used in various multiple access radio wireless communication systems.
OFDM and OFDMA systems generate different channels within a given band by using the orthogonal frequency division multiplexing to generate channel spectral profiles that are orthogonal to one another without interference when different channels are centered at selected equally-spaced frequencies. Under the OFDM, the frequency spacing can be smaller than the minimum spacing in conventional channels and hence increase the number of channels within a given band. The existing and developing specifications under IEEE 806.16x standards support wireless communications under OFDM and orthogonal frequency division multiple access (OFDMA). The drafts for IEEE 806.16e published in January 2004 (revision D3) and revised in May 2005 (revision D8) provide technical specifications for OFDM and OFDMA wireless systems.
One technical feature in OFDM and OFDMA systems is the hand-over process where a mobile subscriber station (MSS) changes from one base station (BS) to another adjacent base station due to various reasons. For example, the hand over may be initiated when the MSS moves in its location due to signal fading, interference levels, etc. at the current serving base station and thus needs to change another base station to which the MSS is connected in order to provide a higher signal quality. In another example, a hand over may be initiated when the MSS can be serviced with higher QoS at another base station.
Such a hand over process may be implemented in different ways. For example, a soft hand over (SHO) process is to operate the MSS to simultaneously communicate with and to receive and send communication traffic with two or more adjacently located base stations and to synchronize the data among the different communication traffic with the different base stations to ensure continuing service during the hand over process.